An optical property measurement apparatus using optical interference has been disclosed in, for example, Patent Documents 1 and 2. An optical property measurement apparatus using a Mach-Zehnder interferometer can measure an optical property of a sample in a flow passage. That is, this type of optical property measurement apparatus causes one of branched light which is branched into two by a first optical coupler to cross a flow passage and causes this branched light and the other branched light to interfere by a second optical coupler to detect intensity of the interfering light, so that the optical property of a sample in the flow passage can be measured.
The optical properties of the sample specifically mean an optical thickness, a refractive index, absorption, and the like. Such data allows an operator to determine size (volume) or dry weight of a sample such as a cell. A phase difference is measured by a measurement unit. Here, it is assumed that change amount of a phase difference φ by passing through a sample is ΔφS. In a case where, for example, it is desired to measure an optical thickness of the sample, the optical thickness of the cell can be calculated by “ΔφS (λ/2π)”. If the refractive index of the sample can be obtained, it is also possible to calculate the particle diameter of the sample. The optical thickness is proportional to the volume in a case where distribution of the refractive index in the sample is uniform, while it is proportional to the dry weight of the sample in a case where the distribution is not uniform (refer to Non-Patent Document 1). Moreover, for the refractive index of the sample, an average refractive index difference (refractive difference between a solvent and the sample) can be calculated by a formula of “Δφs (λ/2π)/D.” Here, D indicates the diameter of the particle. Further, if sum of squares of signals Asin φ and Acos φ which are input into the measurement unit are obtained, an amplitude A is obtained and absorption of the sample can be measured by a degree of attenuation of amplitude intensity. λ indicates wavelength of light.
Patent Document 1: Japanese Patent Publication No. H7-119613
Patent Document 2: US Patent Application Publication No. 2005/0105097
Non-Patent Document 1: E. B. van Munster, Cytometry, Vol. 47, pp. 192-199 (2002).